<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #box{
            width:300px;
            height:200px;
            border:3px solid red;
            overflow: auto;
        }
    </style>
</head>
<body>
    <div id="box">
        <h3>用户须知</h3>
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    </div>
    <input type="checkbox" name="" id="" disabled> 我已阅读
    <button disabled>提交</button>

    <script>
        // 1. 获取元素
        var oBox = document.getElementById('box');
        var oInput = document.querySelector('input');
        var oBtn = document.querySelector('button');

        // max为能够 滚动的最大距离
        var max = oBox.scrollHeight - oBox.clientHeight;

        oBox.onscroll = function(){
            if(oBox.scrollTop >= max - 5){// 还有5像素到底时，就已经到底了
                // 让复选框可选中
                // 让按钮可点击
                oInput.disabled = false;
                oBtn.disabled = false;
            }
        }
    </script>
</body>
</html>